This invention relates generally to gas turbine engines, and, more specifically, to the compression modules therein, such as the fan, booster and the compressor.
In a turbofan aircraft gas turbine engine, air is pressurized in a fan module and a compression module during operation. The air passing through the fan module is mostly passed into a by-pass stream and used for generating the bulk of the thrust needed for propelling an aircraft in flight. The air channeled through the compression module is mixed with fuel in a combustor and ignited, generating hot combustion gases which flow through turbine stages that extract energy therefrom for powering the fan and compressor rotors. The fan, booster and compressor modules have a series of rotor stages and stator stages. The fan and booster rotors are typically driven by a low pressure turbine and the compressor rotor is driven by a high pressure turbine. The fan and booster rotors are aerodynamically coupled to the compressor rotor although these normally operate at different mechanical speeds.
Fundamental in the design of compression systems, such as fans, boosters and compressors, is efficiency in compressing the air with sufficient stall margin over the entire flight envelope of operation from takeoff, cruise, and landing. However, compression efficiency and stall margin are normally inversely related with increasing efficiency typically corresponding with a decrease in stall margin. The conflicting requirements of stall margin and efficiency are particularly demanding in high performance jet engines that require increased auxiliary power extraction, while still requiring high a level of stall margin in conjunction with high compression efficiency.
Operability of a compression system in a gas turbine engine is traditionally represented on an operating map with inlet corrected flow rate along the X-axis and the pressure ratio on the Y-axis, such as for example, shown in FIG. 1 for a fan. In FIG. 1, operating lines 14, 16 and the stall line 12 are shown, along with constant speed lines 22, 24. Line 24 represents a lower speed line and line 22 represents a higher speed line. In conventional designs, as the fan is throttled at a constant speed, such as constant speed line 24, the inlet corrected flow rate decreases while the pressure ratio increases, and the booster operation moves closer to the stall line 12. Furthermore, each operating condition has a corresponding compressor efficiency, conventionally defined as the ratio of actual compressor work input to ideal (isentropic) work input required to achieve a given pressure ratio. The compressor efficiency of each operating condition is conventionally plotted on the compressor map in the form of contours of constant efficiency, such as items 18, 20 shown in FIG. 1. The performance map has a region of peak efficiency, depicted in FIG. 1 as the smallest contour 20, and that for economic reasons it is desirable to operate the compressor in the region of peak efficiency as much as possible.
The operating line of a compressor is set by its downstream throttling orifice. It is known in the art that enlarging the throttling orifice will affect a lowering or drop in operating line 16 while a reduction in orifice area will cause an upward shift of the operating line. In conventional designs the throttle area is varied so as to place the operating line along the ridge of peak efficiency. This is not possible in applications in which the throttle area, such an exhaust nozzle, is fixed and cannot be varied.
In conventional designs, efficiency is usually sacrificed in order to achieve improved operability and increased stall margin. This is particularly challenging to achieve in applications having significant inlet distortion, high altitude operation or high auxiliary power extraction. In applications that require the use of fixed area nozzles, the attainment of adequate stability margin by lowering the component operating lines may seriously compromise engine performance. It is desirable to have a means of improving the stall margin of compression systems apart from simply lowering the operating line and sacrificing efficiency.
Conventional remedies available to improve engine operability include use of overboard bleed and variable position stators. The former is a relatively simple and effective solution, but requires the provision of some overboard bleed pathway. This may be undesirable for some applications wherein penetrations of the vehicle outer mold lines are to be avoided. Variable position stators can be effective at improving stall characteristics, but are mechanically complex and expensive.
In order to avoid a stall, it is desirable in fans, boosters and compressors in a gas turbine engines with fixed nozzle aperture to have means to control the operating line such that sufficient stall margin with respect to the stall line 12 is maintained before a stall initiates without sacrificing efficiency, for a wide operating range. It is desirable to have a method and means for controlling the operating line of a compression system having a fixed area throttling orifice downstream. It is desirable to have a compression system with operating lines to affect highest possible cruise performance and still have high levels operability over a wide range of operating conditions of altitude, power extraction, and inlet distortion.